1. Field of the Invention
The invention relates to semiconductor wafer processing equipment and, more particularly, the invention relates to electrostatic substrate supports having an RF bias electrode.
2. Description of the Background Art
A semiconductor wafer processing system for manufacture of integrated circuits (IC's) generally includes a vacuum chamber within which is mounted a wafer support during processing. The wafer support typically comprises a susceptor mounted to a pedestal. The pedestal is typically fabricated from a metal such as aluminum. The susceptor may be fabricated from laminated sheets of a polymer. However, for high temperature applications, the susceptor is typically fabricated from a ceramic material such as aluminum oxide or aluminum nitride. The susceptor typically contains various components which provide heating and/or cooling of the wafer. The susceptor may also include elements for clamping (chucking) a wafer to retain it in a stationary position upon the susceptor surface. Such clamping is provided by either a mechanical clamp or an electrostatic chuck. The susceptor may also include one or more electrodes for applying a bias voltage to the wafer. Such a bias voltage may be a direct current (DC) bias or a radio frequency (RF) bias. An RF bias may be used, for example, to supply or enhance power to a plasma that exists within the chamber during an etch or deposition process.
FIG. 1 depicts a wafer support 100 of the prior art. In the wafer support 100, a pedestal 102 supports a ceramic susceptor 104. The susceptor 104 is typically made by cold laminating several layers 106.sub.i (e.g. layers 106.sub.1, 106.sub.2 . . . 106.sub.5) of "green tape" consisting of a ceramic powder of alumina or aluminum nitride with a suitable organic binder such as butadiene (synthetic rubber) or poly-methyl methacrylate. The electrode patterns 108 are screen or stencil printed onto the appropriate green tape layer using inks or pastes made with molybdenum or tungsten powders along with a suitable organic binder. Interconnections between the electrodes 108 and connections 112 to the exterior of the susceptor are made by punching holes in the green tape layers and filling them with the same or similar tungsten/molybdenum paste through screen printing masks to form vias 110. The laminated layers are sintered at elevated temperatures to solidify the ceramic to form the monolithic susceptor 104 with thick film metal electrodes 108 embedded in the ceramic. During the sintering process, the organic binders are charred and removed as CO and CO.sub.2.
The metal paste and the ceramic layers 106 generally sinter at different temperatures and, thus, shrink non-uniformly during sintering. Such non-uniformity in the shrinkage, if severe enough, causes severe bowing, distortion or cracking of the laminate. To alleviate such problems, the metal inks or pastes are usually mixed with large proportions (up to 40% by volume) of ceramic powder to match the shrinkage behavior of the surrounding ceramic during sintering.
Prior art attempts to solve the problem include using thicker electrodes. However, the thickness of the electrodes 108 is limited to only 10 or 15 microns by the sintering process. If the metal electrodes are made thin and fragile they easily break and reform many times due to the shrinkage in the surrounding ceramic. Thus, strains are dissipated before they become large enough to damage the ceramic.
The resulting electrodes 108 and interconnecting vias 110 and connections 112 are, therefore, highly resistive. High resistivity is not a problem where the electrode 108 serves only as a DC electrode for chucking or DC bias. However, high resistivity presents a problem when it is desired to use the electrode 108 for RF bias.
The problem arises because, in common designs, the vias 112 connecting the electrodes 108 to the outside are located in the central area 114 of the susceptor. A thin highly resistive electrode will have a high impedance due to ohmic resistance. Consequently, a large proportion of the power delivered to the electrodes 108 will be dissipated as heat. Thus, the efficiency of RF power transferred to the plasma in the chamber is low. This is unsuitable for delivery of RF energy to the plasma at power levels of 500 to 1500 watts. Furthermore, the RF power distribution over the area of electrodes 108 is non-uniform which leads to non-uniformity of the plasma temperature across the wafer. The plasma develops "hot spots" where the plasma temperature is greater. Consequently, the etch, deposition or other plasma process will be non-uniform over the surface of the wafer. Thus, a number of the IC's on a given wafer may be rendered unusable thereby decreasing wafer yield.
Other solutions involve stacking multiple thin RF electrodes in layers separated by layers of ceramic. The RF power is then capacitively coupled from one electrode layer to the next. Although this does reduce the overall impedance of the metal structure, it also leads to power losses through the capacitive couplings. Furthermore, it does not solve the problem of non-uniform RF power distribution over the area of the electrodes.
Therefore, a need exists in the art for a susceptor for a semiconductor processing system having a low impedance electrode structure that uniformly distributes RF power over the area of the electrode and a concomitant method of manufacturing same.